Add LOCAL_SOURCE_FILES_ALL_GENERATED support

Set "LOCAL_SOURCE_FILES_ALL_GENERATED := true" if you want to build a
Java library from only generated source files, without any static
source files. Previously this is not allowed.
This is needed by building api stub libraries, which have only source
files of generated stub files.

Bug: 6814443
Change-Id: Idb2ae9c71a473b796154a03ef07a3403461c4f3c